1. Field of the Invention
This invention lies in the field of hoisting machinery, or cranes.
More particularly it concerns the design of extensible booms for cranes.
Still more particularly, it concerns the design of an extensible boom for a crane in which the extension of the two parts of the boom is controlled by a tension member, rather than a compression member, as in conventional designs.
2. Description of the Prior Art
In the prior art, extensible booms are made with at least two linear members, one of which is adapted to be guided and slide into and out of a second member, generally the outer of the two. The outer part of the boom is anchored at a rotatable pedestal by means of a horizontal shaft so that the boom can be raised and lowered by appropriate cable means.
In the prior art, the extension of the inner member of the beam, with respect to the outer member is generally by means of a hydraulic piston and cylinder in which the piston rod is of sufficient diameter and strength to operate as a compression member, in extending the inner part of the boom. This requires a rather costly and heavy mechanism for controlling the relative extension of the inner member with respect to the outer member.